There are numerous methods for the manufacturing of gear wheels. In the chip-cutting soft pre-machining, one distinguishes between hobbing, gear shaping, generating planing and skiving (also called power skiving). The hobbing and skiving are so-called continuous methods, as will be explained in the following in more detail.
In the chip-cutting manufacturing of gear wheels, one distinguishes between the intermitted indexing process (or also called single indexing process) and the continuous method, which is partly also called continuous indexing process or face hobbing.
In the continuous method, for example, a tool comprising corresponding cutters is applied in order to cut the flanks of a work piece. The work piece is being cut ready continuously, i.e., in an uninterrupted method, in one clamping. The continuous method is based on complex, coupled movement sequences, in which the tool and the work piece to be manufactured perform a continuous indexing motion relative to each other. The indexing movement results from the coordinated, respectively coupled driving of several axle drives of a corresponding machine.
In the single indexing process, one tooth gap is manufactured, then for example a relative movement of the tool and a so-called indexing movement (indexing rotation), in which the work piece rotates relative to the tool, are carried out, before then the next tooth gap is manufactured. A gear wheel is thus manufactured step by step.
The gear shaping method mentioned initially can be described or represented by a cylindrical gear transmission system, because the intersection angle (also called intersection angle of axes) between the rotation axis R1 of the shaping tool 1 and the rotation axis R2 of the work piece 2 amounts to zero degree, as represented schematically in FIG. 1. The two rotation axes R1 and R2 run parallel to each other, if the intersection angle of axes amounts to zero degree. The work piece 2 and the shaping tool 1 rotate continuously about their rotation axes R2 and R1, respectively. In addition to the rotary movement, the shaping tool 1 performs a stroke movement, which is referenced by the double arrow shx in FIG. 1, and removes chips from the work piece 2 during this stroke movement.
Some time ago, a method was developed, which is called skiving. The basics are approximately 100 years old. A first patent application having the number DE 243514 on this subject dates back to the year 1912. After the initial considerations and investigations of the starting years, skiving was no longer seriously pursued further. At that time, complex processes, which were partly empirical, were necessary in order to find a suitable tool geometry for the skiving method.
About in the middle of the nineteen-eighties, skiving was taken up again. It was not until the present-day simulation methods and the modem CNC-controls of the machines, that the principle of skiving could be implemented in a productive, reproducible and robust method. The high wear resistance of present-day tool materials, the enormous high static and dynamic rigidity and the high performance of the synchronous running of modem machines aid in skiving.
In skiving, as shown in FIG. 2A, an intersection angle of axes Σ between the rotation axis R1 of the skiving tool 10 (also called skiving wheel) and the rotation axis R2 of the work piece 20 are predetermined, where the angle is different from zero. The resulting relative movement between the skiving tool 10 and the work piece 20 is a helical movement, which can be divided into a rotation portion (rotatory portion) and an advance portion (translative portion). A generation helical type gear transmission may be considered as a drive technology-related analogon, wherein the rotatory portion corresponds to the rolling and the translative portion corresponds to the gliding of the flanks. The greater the absolute value of the intersection angle of axes Σ, the greater translative movement is required for the manufacturing of the work piece 20. It effects namely a movement component of the cutting edges of the skiving tool 10 in the direction of the tooth flanks of the work piece 20. Thus, when skiving, the gliding portion of the relative combing movement of the engaging gear wheels of the equivalent helical gear transmission is utilized to perform the cutting movement. Only a slow axial advance is required in skiving and the so-called pushing movement, which is typical for the gear shaping, is dispensed with. Thus, a reverse stroke movement does not occur in skiving.
The cutting speed in skiving is influenced directly by the rotation speed of the skiving tool 10 with respect to the work piece 20 and by the utilized intersection angle of axes Σ of the rotation axes R1 and R2. The intersection angle of axes Σ, and thus the gliding portion, should be selected such that for the machining of the material, an optimum cutting speed is achieved for a given rotation speed.
The motion cycles and further details of a previously known skiving method can be taken from the previously mentioned schematic representation of FIG. 2A. FIG. 2A shows the skiving of an exterior tooth system on a cylindrical work piece 20. The work piece 20 and the tool 10 (here: a cylindrical skiving tool 10) rotate in opposite directions.
Further relative motions occur. An axial feed sax is required in order to be able to machine the entire tooth system width of the work piece 20 with the tool 10. If a helical gearing is desired on the work piece 20 (i.e. β2≠0), then a differential feed SD is superimposed on the axial feed sax. A radial feed Srad may be employed in order to influence the crowning of the tooth system of the work piece 20.
In skiving, the vector of the cutting speed νc results essentially as the difference of the two velocity vectors ν1 and ν2 of the rotation axes R1, R2 of the tool 10 and the work piece 20, which axes are slanted with respect to each other by the intersection angle of axes Σ. Here, ν1 is the velocity vector at the periphery of the tool 10 and νc is the velocity vector at the periphery of the work piece 20. Thus, the cutting speed νc of the skiving process can also be changed by the intersection angle of axes Σ and the rotation speed in the equivalent helical gear transmission. The axial feed sax has only a small influence on the cutting speed νc, which can be neglected and is therefore not shown in the vector diagram with the vectors ν1, ν2 and νc in FIG. 2A.
The skiving of an outer tooth system of a work piece 20 using a conical skiving tool 10 is shown in FIG. 2B. In FIG. 2B, the intersection angle of axes Σ, the vector of the cutting speed νc the velocity vectors ν1 at the periphery of the tool 10 and ν2 at the periphery of the work piece 20 are shown again as well as the helix angle β1 of the tool 10 and the helix angle β2 of the work piece 20. In FIG. 2A, the helix angle β2 is different from zero. The tooth head of the tool 10 is referenced with the reference numeral 4 in FIG. 2B. The tooth breast is referenced with the reference numeral 5 in FIG. 2B. The two rotation axes R1 and R2 do not intersect each other, but are arranged skew relative to each other. In a conical skiving tool 10, the calculation point AP is heretofore commonly chosen to be on the joint plumb of the two rotation axes R1 and R2, because a bending of the skiving tool 10 for providing relief angles is not necessary. The calculation point AP coincides with the so-called contact point here. The rolling circles of the equivalent helical gear transmission contact each other in this calculation point AP.
It is known from the German patent application DE 3915976 A1 that in skiving one can superimpose a radially directed movement on the slow axial feed in order to achieve modifications of the flank line. In this way, the crowning of a tooth system can be influenced.
In addition, it is known from the international patent application WO 2010/060733 A1 that by superimposing the axial feed with a radial movement, tooth systems can be generated, the tooth grooves of which phase out at the respective end radially and axially into the surface of the untoothed work piece. The mentioned international patent application concerns primarily a so-called multi-cut strategy, as indicated schematically in FIG. 3. In FIG. 3, the phasing out of a tooth groove 7 of a corresponding work piece 8 is shown. FIG. 3 shows several traces, which the skiving tool has formed in the work piece 8 during the multi-cut strategy. Due to the superimposition of the axial feed with the radial movement, a phasing out profile results at the end of the tooth groove 7, where the profile is composed of several curved segments in the area 9. The mentioned international patent application does not address the generation of completely generated tooth gaps.
In order to increase skiving productivity as much as possible, for example when utilizing modern cutting materials such as hard metals for the dry machining, the glide portion of the relative movement between the skiving tool and the work piece must generate sufficiently high cutting speeds. In skiving, the cutting speed νc is influenced directly by the rotation speed of the equivalent helical gear transmission, by the effective radii of the work piece with respect to the tool and by the intersection angle of axes Σ of the rotation axes R1 and R2. The possible rotation speed is limited here by the allowed rotation speeds of the machining apparatus (skiving machine) utilized. The size of the work piece is fixedly predetermined. The possible size of the tool is limited by the working space of the machining apparatus (skiving machine) and for inner tooth systems also by the inner space of this tooth system itself. Thus, sufficiently high cutting speeds can often be generated only by correspondingly high intersection angle of axes Σ.
In skiving, a tool 10 comes to application, which comprises at least one geometrically determined cutting edge. The cutting edge/edges are not shown in FIGS. 2A and 2B. The shape and arrangement of the cutting edges belong to those aspects, which must be taken into account in a concrete design in practice.
In addition, in skiving, the tool itself inheres great relevance. In the example shown in FIG. 2A, the skiving tool 10 has the shape of the straight-toothed spur wheel. The outer contour of the base body in FIG. 2A is cylindrical. However, it may also be bevel-shaped (also called conical) as shown in FIG. 2B. Because the one or plural teeth of the skiving tool 10 engage along the entire length of the cutting edge, each tooth of the tool 10 at the cutting edge requires a sufficient relief angle.
Starting from a straight-toothed or a helically toothed conical skiving tool 10, as shown in the FIGS. 4A and 4B, one recognizes that such a skiving tool 10 has so-called constructional relief angles due to the conical base shape of the skiving tool 10. That is the relief angles at the head and at the flanks of the conical skiving tool 10 are predetermined due to the geometry of the skiving tool 10. However, the profile of the cutting edges of a conical skiving tool 10, must satisfy certain conditions in order to actually enable a regrinding. In FIGS. 4A and 4B, a conical skiving tool 10 is shown during the cutting of outer teeth on a work piece 20. The so-called constructional relief angle αKo at the cutter head of the conical skiving tool 10 is visible in FIG. 4B. The intersection point of axes AK and the contact point BP of the rolling circles of the skiving tool 10 and the work piece 20 coincide in FIG. 4A and lie on the joint plumb GL (not visible respectively shown in the FIGS. 4A and 4B) of the rotation axes R1 and R2.
In FIG. 5, a further illustration of a straight-toothed or a helically toothed conical skiving tool 10 and cylindrical work piece 20 is shown, wherein the view of FIG. 5 has been chosen such that both rotation axes R1 and R2 extend parallel, although the two axes R1 and R2 are skew with respect to each other. In FIG. 5, the joint plumb GL of the two axes R1 and R2 is visible. The contact point BP lies on the joint plumb GL as shown in FIG. 5.
In the FIGS. 6A and 6B, a constellation of a cylindrical skiving tool 10 and a cylindrical work piece 20 comprising outer teeth is shown. The skiving tool 10 is not only arranged skew with respect to the rotation axis R2 of the work piece 20 (as can be recognized in FIG. 6A on the basis of the corresponding intersection angle of axes Σ), but is positioned with respect to the work piece 20 such that it is also inclined away from it by a small angle αKi (as can be seen well in FIG. 6B). By the insetting-in away of the skiving tool 10, an effective relief angle can thus be generated, which is shown in FIG. 6B for the head cutting edge as αKi. Also at the side cutting edges of the tool, effective relief angles are generated by the insetting-in away. However, these turn out to be smaller than on the head cutting edge. Generally, these relief angles are only half as large.
Starting from a straight-toothed or a helically toothed cylindrical skiving tool 10, as shown in the FIGS. 6A and 6B, one recognizes that such a skiving tool 10 does not have so-called constructional relief angles, neither at the head nor at the flanks. If such a cylindrical skiving tool 10 was clamped in the conventional manner, no relief angles would be provided. By the insetting-in away of the skiving tool 10, a kinematic relief angle can be generated, as already described. In practice, the insetting-in away of the skiving tool 10 is achieved by an eccentric clamping of the skiving tool 10 in the machine, in order to thus effect an offset of the cutting face from the intersection point of axes AK. Due to the insetting-in away of the skiving tool 10, the contact point BP of the rolling circles of the skiving tool 10 and the work piece 20 no longer lies on the joint plumb of the rotation axes R1 and R2. The corresponding offset is also called cutting face offset e and can be recognized in FIG. 6A. The further the skiving tool 10 is inclined away, the greater the effective relief angles become. The relief angles required for skiving are in the range between 3 degrees and 5 degrees. In order to prescribe these relief angles, an insetting-in away of cylindrical skiving tools 10 of up to 10 degrees is required and common in practice.
In FIGS. 7A and 7B, further illustrations of a straight-toothed or a helically toothed cylindrical skiving tool 10 and a cylindrical work piece 20 are shown, wherein the view in FIG. 7A has been chosen such that the two rotation axes R1 and R2 extend parallel, although the two axes R1 and R2 are skew with respect to each other. In FIG. 7A, the joint plumb GL of the two axes R1 and R2 can be seen. In the FIGS. 7A and 7B, the contact point BP is located above the joint plumb GL. In FIG. 7B a so-called contact view (also called side projection of contact plane) is shown, in which the contact point BP is visible. In the representation of FIG. 7A, the contact point BP is hidden behind the work piece 20.
Own investigations of previous skiving methods have shown, that a sudden failure of the skiving tool may occur. More detailed considerations and evaluations have shown that extremely negative rake angles may occur during the skiving, among others. Simulations of the totality of the trajectory points of the cutting edges, which cut into the material of the work piece have shown that in the complete cutting of the gap, the effective head rake angle becomes particularly more and more negative from the beginning of the chip formation up to the exit of the skiving tool from the gap. That is more precisely, during the ablation of a chip in the common skiving, the chip thickness increases starting from the generator line, wherein the effective rake angle decreases continuously starting from approximately zero degree initially. At the end of the formation of a chip at the cutter head, this effective rake angle may amount to for example up to −60 degrees or in very unfavorable cases even less than −60 degrees. This aspect may lead to a premature wearing of the skiving tool.
The movement of the cutting tooth 6 of a skiving tool 10 through the material of a work piece 20 is represented schematically in the FIGS. 8A to 8C. The FIGS. 8A to 8C show the effective progression of the rake angle at the cutter head, respectively at the cutting tooth 6, over the course of the cut during the complete cut. Due to the superimposition of the coupled, i.e. mutually synchronized rotational motions of the skiving tool 10 about the first rotation axis R1 and of the work piece about the second rotation axis R2, and of linear axial movements of the skiving tool 10 relative to the work piece 20, in a record of the totality of the trajectory points of a cutter, there result a sort of trough- or hutch-shape, as shown in FIGS. 8A to 8C, 9, 10 and 11A, 11B. In FIGS. 8A to 8C, 9 and 10, the corresponding trough is referenced with the reference numeral 11.
FIG. 8A shows the relative movement of the cutting tooth 6 of the skiving tool 10 in the material of the work piece 20 in a first snap shot. The orientation and position of the cutting edge 6.1 of the cutting tooth 6 is represented by a thick line. The trough 11 results from the totality of the trajectory points of the cutting edge 6.1 of the cutting tooth 6 lying in the gap 22 of the work piece 20 for one engagement of the cutting tooth 6 in this gap 22. The subsequent engagement of a further cutting tooth (this may be the same or another cutting tooth of the tool) also generates a trough 11, which is offset within the gap 22 in the axial direction due to the axial feed and the differential feed coupled therewith. Thus, the trough 11 moves stepwise (at least virtually) through the material of the work piece 20 during skiving. FIG. 8A shows a line 12 which divides the trough 11 in a left and a right section. The masked portion of the line 12 is shown as a dashed-dotted line. The line 12 delimits the superposition of two troughs from each other, which differ in their position by the feed between two directly successive engagements of cutting teeth. That is, the line 12 characterizes the intersection curve of the two troughs. For an infinitesimally small axial feed, this intersection curve corresponds to the so-called generator line. The total tooth gap can be considered as a set of such generator lines, which run through the material of the work piece 20 in the cutting direction. In the conventional skiving machining process comprising an axial feed, material is ablated by the driven cutting edge 6.1 from the work piece 20 only in the section starting from the generator line (i.e. in the concretely shown representation: to the left of the generator line). Material has already been ablated by the previous engagement of the tool in the cutting direction before the generator line (i.e. in the concretely shown representation: to the right of the generator line).
FIG. 8B shows a second snap shot of the simulation, wherein the cutting tooth 6 of the skiving tool 10 has moved in the material of the work piece 20 with respect to the situation in FIG. 8A by a distance further to the left in the cutting direction SR.
FIG. 8C shows a third snap shot of the simulation, wherein the cutting tooth 6 of the skiving tool 10 has moved in the material of the work piece 20 with respect to the situation in FIG. 8B still a distance further to the left in the cutting direction SR. In FIG. 8C, it is clearly visible that the cutting face of the cutting edge 6.1 of the cutting tooth 6 forms an acute angle with respect to the trough 11. The corresponding “critical” section is characterized in FIG. 8C with the reference numeral 13. Hence an extremely negative effective rake angle results in the section 13, as already mentioned.
During the skiving process, the effective rake angle of the head becomes particularly more and more negative, as already mentioned. In skiving, the blade angle of the cutting tooth 6 to the imagined bottom of the gap at the work piece 20 remains approximately constant. The head cutting edge of the cutting tooth 6 “grinds” over the bottom of the trough.
A section of a work piece 20 comprising plural teeth 21 and tooth gaps 22 is shown in FIG. 9. Now, if one considers the total progression of the movement in the skiving machining of a work piece 20, it is visible that the trough 11 moves through a tooth gap 22 until the tooth gap 22 is completely accomplished. The movement of the trough 11 through the tooth gap 22 is indicated by a directional arrow VR, which points in the feed direction. This feed direction is composed of the axial and the differential feed.
The above-mentioned investigations of the rake angle during the skiving hold in particular for the generation of a complete chip in the ongoing skiving process. However, they also provide important insights on the start of the skiving process, at which one must first “pierce” into the gap. In the common infeed of the skiving tool in the axial direction of the work piece, a first contact of the cutting tooth 6 with the work piece 20 occurs with a clearly negative effective rake angle. Thus, previously, the first chips are generated with conceivable bad chip cutting conditions. Due to the large negative rake angle at the first contact, the load on the cutting edge 6.1 is very high. The forces acting on the cutting edge 6.1 (in particular the edge of the cutting edge) thereby increase suddenly, which may lead to an immediate destruction of the cutting edge 6.1. This high load may explain the sudden, abrupt failure of the skiving tool 10 which has partly been observed. In the ongoing skiving process according to FIGS. 8A to 8C and 9, the adverse effective rake angles do also occur as mentioned above; however, the load on the cutting edge 6.1 is established continuously and not abruptly hereby. Thus, different wear or failure phenomena are concerned.
In the axial infeed which has been practiced previously, the trough 11 is guided at the side along the tooth gap 21 such that trough edge of the trough 11 having the worst cutting conditions touches the work piece 20 for the first time in the section of the front side 23 as shown in FIG. 10. This first contact occurs typically with the head section of the cutting edge 6.1 at the side of the feathering flank. The axial infeed is indicated in FIG. 10 by the arrow ZB. Hereby, the arrow ZB runs parallel to the rotation axis R2 of the tool 20.